Common drug in wastewater is surprising reason behind gender-bending fish

Move over birth control pills, there's a new drug in town making male fish bear eggs.

Birth control pills and herbicides in water are known to wreak all kinds of havoc on the sexual development of aquatic life and possibly even our children, but now a researcher from University of Wisconsin-Milwaukee has found another culprit. The medicine metformin, commonly taken for Type II diabetes and found in freshwater worldwide, has been shown to cause intersex fish – males that can produce eggs. Male fish are not supposed to be able to produce eggs.

Pharmaceuticals are increasingly found in wastewater and surface waters around the world, often due to incomplete metabolism in humans and subsequent excretion in human waste, resulting in discharge into surface waters by wastewater treatment plant effluent.

Since intersex fish are especially common downstream from wastewater treatment plants, there has been a lot of research looking into the effect of hormones from birth control pills, said researcher, Rebecca Klaper.

But looking at chemicals prevalent in water samples collected from Lake Michigan, Klaper came across high levels of metformin. In fact, it was the pharmaceutical with the highest levels in the samples.

"It is the chemical we found in almost every sample and in the highest concentrations compared to other emerging contaminants – even higher than caffeine," Klaper said.

"I was kind of a surprised," she said. "It was not even on our radar screen. I said, 'What is this drug?' "

Klaper set out to see what effects it was potentially having on the environment. What she found was that in addition to intersex conditions in fathead minnows, fish exposed to metformin were smaller than fish that were not exposed.

This was surprising since metformin targets blood sugar regulation and is not a hormone.

Upon further investigation, Klaper’s research revealed that in fact, metformin could be a potential endocrine disruptor. Noting that it is also prescribed to women with the hormonal disease, polycystic ovary syndrome.

Metformin is the most commonly prescribed medicine for Type II diabetes. In 2013, some 70 million prescriptions were dispensed with sales of more than $23 billion. And with diabetes rates skyrocketing – between 1980 and 2008, the number of diabetics worldwide more than doubled from 153 million to 347 million – the gender-bending drug will likely become ever more prevalent.